1. Field of the Invention
The present invention relates to an auto-focusing device of a camera, and more particularly to a tracking-drive of a photographing lens to always keep the photographing lens in focus to a moving object.
2. Related Background Art
An auto-focusing device having a so-called overlapped servo function which servos a photographing lens into an in-focus position (hereinafter referred to as an AF servo) during a charge accumulation period of a charge accumulation type photo-electric conversion element (hereinafter referred to as an AF sensor) to enhance the tracking ability to a moving object has been known. (For example, Japanese Laid-Open Patent Application No. 2-146010).
The assignee of the present invention has proposed in (Japanese Patent Application No. 2-256677) an auto-focusing device having a so-called overlap prediction drive function which detects the movement of an object while conducting overlapped servo and predicts a position of the object to drive a photographing lens.
FIG. 11 illustrates the overlap prediction drive method. An abscissa represents a time t and an ordinate represents a distance Z on an optical axis. A curve Q represents a distance on the optical axis between the photographing lens and a focusing position of the object and it varies as the object moves. On the other hand, a curve L represents a distance on the optical axis between the photographing lens and a film plane. Accordingly, a difference between Q and L represents a defocus amount D. Times such as t(n) and t(n-1) are substantially center times of accumulation times of the charge accumulation type AF sensor, and time periods surrounded by lines parallel to the ordinate Z on the opposite sides of the center times represent charge accumulation times. D(n-1) and D(n) denote defocus amounts at the times t(n-1) and t(n), respectively. Hereinafter, the distance measurement time is represented by the time t(n) although the charge accumulation time for measuring the distance is actually required so long as the charge accumulation type AF sensor is used. In the present specification, the measurement of distance means the charge accumulation of the charge accumulation type AF sensor. In a drawing which illustrates the overlapped servo, the ordinate, the abscissa and the curves Q and L are identical to those of FIG. 11.
As seen from FIG. 11, a displacement P(n) of the focusing plane of the object from the time t(n-1) to the time t(n) is determined from the defocus amount D(n) detected by the measurement of distance at the time t(n), the previous defocus amount D(n-1) and a displacement M(n) of the photographing lens between this period. EQU P(n)=D(n)+M(n)-D(n-1) (1)
Accordingly, an image plane velocity S(n) of the object is given by EQU S(n)=P(n)/{t(n)-t(n-1)} (2)
Assuming that the distance measurement period {t(n+1)-t(n)} is substantially the same for each time, a displacement of the object from the time t(n) to the next distance measurement time t(n+1) may be predicted as P(n).
A data transfer time of a CCD and a calculation time for the defocus amount are included from the termination of the charge accumulation by the AF sensor to the calculation of the defocus amount. In the overlapped servo, the photographing lens is usually driven even during this period. Thus, when the lens drive distance is to be determined based on the calculated defocus amount, a correction must be made by taking the displacement of the photographing lens during this period into account. Assuming that the end time of calculation of the defocus amount by the distance measurement at the time t(n) is tm(n) and the displacement of the photographing lens from the time t(n) to the time tm(n) is PD(n), a lens drive distance at the time tm(n), that is, a total drive distance V(n) to be servoed such that the defocus amount at the next distance measurement is zero is calculated in the following manner. Namely, the total drive distance V(n) is calculated by adding the defocus amount D(n) measured at the time t(n) to the predicted displacement P(n) of the object from the time t(n) to the next distance measurement time t(n+1), and subtracting therefrom the lens drive distance PD(n) from the time t(n) to the defocus amount calculation end time tm(n). ##EQU1##
If the photographing lens is driven along the curve L1 in accordance with the total drive distance V(n) calculated by the formula (3) during the period from the calculation end time tm(n) to the start of the next distance measurement, the defocus amount D(n+1) at the next measurement distance is substantially zero. However, since the next distance measurement is started soon after the calculation of the defocus amount at the time tm(n), the period from the time tm(n) to the start of the next distance measurement is very short, and it is difficult to drive the photographing lens during that period because of a limit of a motor power. The photographing lens is usually driven along a broken curve L2 and the drive of the lens by the total drive amount V(n) calculated by the formula (3) is not completed by the start of the next distance measurement. As a result, the photographing lens cannot catch up to the moving object even if the control is updated by using the total drive distance V(n) of the formula (3) as a servo target each time the defocus amount is calculated.
In order to solve this problem, in Japanese Patent Application No. 2-256677 mentioned above, the correction amount to be subtracted when the total drive distance V(n) is calculated, that is, the lens displacement PD(n) from the time t(n) to the time tm(n) is ignored, and the total drive distance V(n) calculated by ##EQU2## is servoed. As a result, as shown in a curve L3, the photographing lens is driven in excess of PD(n) from the predicted position of the object at the time t(n+1) so that the photographing lens approaches more closely to the curve Q.
However, the latter auto-focusing device described above includes the following problem.
In the prior art device, since the displacement of the lens during the calculation period of the defocus amount is not taken into account in calculating the lens drive distance, the photographing lens is driven in excess and the photographing lens finally passes the object at a certain distance measurement time. Since the lens drive distance during the calculation period of the defocus amount is not taken into account at the next update of the servo, the photographing lens finally passes the object by a fairly long distance and the total drive distance calculated by the formula (4) becomes negative, when the lens drive is first stopped until a normal condition for the photographing lens to follow the object is recovered. As a result, the photographing lens passes the object and stops, and passes the object and stops, and makes an unstable and discontinuous movement.
It is an object of the present invention to provide an auto-focusing device which stably drives a photographing lens to track a moving object.
One aspect of the present invention as shown in FIG. 1A relates to an auto-focusing device that comprises charge accumulation type photo-electric conversion means 101 for accumulating charges at a predetermined time interval in accordance with a focus detection light beam transmitted through a photographing lens to produce a focus detection signal; defocus amount calculation means 102 for calculating a defocus amount including a deviation between a focus plane of the focus detection light beam transmitted through the photographing lens and an anticipated focus plane and a direction of the deviation based on the focus detection signal after the end of each accumulation of the charge by the photo-electric conversion means 101; drive distance calculation means 103 for calculating a direction and a distance of lens drive for driving the photographing lens to track a moving object based on at least the defocus amount produced by the defocus amount calculation means 102; drive means 104 for driving the photographing lens in accordance with the direction and the distance of the lens drive calculated by the drive distance calculation means 103.
It further comprises control means 105 for supplying the direction and the distance of the lens drive calculated by the drive distance calculation means 103 to the drive means 104 after the end of the next charge accumulation by the photo-electric conversion means 101 to drive the photographing lens.
The control means 105 supplies the direction and distance of the lens drive calculated by the drive distance calculation means 103 to the drive means after the end of the next charge accumulation by the photo-electric conversion means 101 to drive the photographing lens.
In accordance with this aspect, the direction and distance of the lens drive to drive the photographing lens to track the moving object are calculated based on at least the defocus amount calculated after the charge accumulation, and the photographing lens is driven after the end of the next charge accumulation in accordance with the calculated direction and distance of the lens drive. Accordingly, the photographing lens can stably track the moving object and the tracking ability is improved.
Another aspect of the present invention comprises drive distance calculation means 103A for calculating the direction and the distance of the lens drive for making the photographing lens in focus at the (N+2)th charge accumulation based on the defocus amount calculated at the end of the at least N-th charge accumulation by said photo-electric conversion means 101. The drive distance calculation means 103A calculates the direction and distance of the lens drive for making the photographing lens in focus at the (N+2)th charge accumulation based on at least the defocus amount calculated at the end of the N-th charge accumulation of the photo-electric conversion means 101.
In accordance with the second aspect, the direction and distance of the lens drive for making the photographing lens in focus at the (N+2)th charge accumulation are calculated based on at least the defocus amount calculated at the end of the N-th charge accumulation. Accordingly, the photographing lens may be driven over a relatively long period from the end of the (N+1)th charge accumulation to the start of the (N+2)th charge accumulation, and the tracking ability is improved as it is in the first aspect.
A third aspect of the present invention comprises drive distance calculation means 103B for calculating the direction and the distance of the lens drive by predicting a displacement of the object during a release delay time from the release of a shutter to the light exposure to a film. The drive distance calculation means 103B calculates the direction and distance of the lens drive by predicting the displacement of the object during the release delay time from the shutter release to the light exposure to the film.
In accordance with the third aspect, the direction and distance of the lens drive are calculated by predicting the displacement of the object during the release delay period from the shutter release to the light exposure to the film. Accordingly, the photographing lens can be brought to an exact in focus position at the time of exposure for a fast moving object.
A fourth aspect of the present invention comprises drive distance calculation means 103C for calculating the direction and the distance of the lens drive by correcting a portion of the predicted displacement of the object during the release delay period. The drive distance calculation means 103C calculates the direction and distance of the lens drive by correcting a portion of the predicted displacement of the object during the release delay period.
In accordance with the fourth aspect, the direction and distance of the lens drive are calculated by correcting a portion of the predicted displacement of the object during the release delay period. Accordingly, a similar effect to that of the third aspect is attained.
A fifth aspect of the present invention comprises control means 105A which does not output current direction and distance of the lens drive to the drive means 104 when the current direction of the lens drive calculated by the drive distance calculation means 103-103C is different from the previous direction of the lens drive. The control means 105A does not output the current direction and distance of the lens drive to the drive means 104 when the current direction of the lens drive calculated by the drive distance calculation means 103-103C is different from the previous direction of the lens drive.
In accordance with the fifth aspect, the photographing lens is not driven in accordance with the currently calculated direction and distance of the lens drive when the currently calculated direction of the lens drive is different from the previous direction. Accordingly, the photographing lens is driven smoothly.
A sixth aspect of the present invention as shown in FIG. 1B relates to an auto-focusing device comprising: charge accumulation type photo-electric conversion means 201 for accumulating charges at a predetermined time interval in accordance with a focus detection light beam transmitted through a photographing lens to produce a focus detection signal; defocus amount calculation means 202 for calculating a defocus amount including a deviation between a focus plane of the focus detection light beam by the photographing lens and an anticipated focus plane and a direction of the deviation based on the focus detection signal after the end of each accumulation of the charge by the photo-electric conversion means 201; first drive distance calculation means 203 for calculating a direction and a distance of lens drive to drive the photographing lens to track a moving object based on at least the defocus amount produced by the defocus amount calculation means 202; lens displacement detection means 204 for detecting actual direction and displacement of the photographing lens; drive means 205 for driving the photographing lens and in accordance with the direction and the distance of the lens drive calculated by the first drive distance calculation means 203.
It further comprises displacement accumulation means 206 for accumulating the direction and the displacement of the photographing lens detected by the lens displacement detection means 204; and second drive distance calculation means 207 for adding the accumulation of the direction and the displacement accumulated by the displacement accumulation means to the direction and distance of the lens drive calculated by the first drive distance calculation means 203. The drive means 205 drives the photographing lens in accordance with a lens drive target calculated by the second drive distance calculation means 207.
In accordance with the sixth aspect, the direction and distance of the lens drive to drive the photographing lens to track the moving object are added to the accumulation of the actual direction and displacement of the photographing lens to calculate the lens drive target for driving the photographing lens. Accordingly, the positions of the photographing lens at the respective times may be represented on a common scale, the calculation of the lens drive distance in the auto-focusing control is simplified, and the control response is improved.
A seventh aspect of the present invention comprises lens displacement detection means 204A including an encoder for generating a pulse signal for each predetermined displacement of the photographing lens, displacement accumulation means 206A including a counter for counting pulse signals supplied from the lens displacement detection means 204A to accumulate the direction and the displacement of the photographing lens, and second drive distance calculation means 207A for converting the direction and the distance of the lens drive calculated by the first drive distance calculation means 203 to a number of pulses per predetermined displacement and adding the pulse count counted by the displacement accumulation means 206A to the number of pulses to calculate the lens drive target.
In accordance with the seventh aspect, the direction and displacement of the photographing lens are detected by the encoder and the pulse signals from the encoder are counted to accumulate the actual direction and displacement of the photographing lens, and the direction and distance of the lens drive converted to the number of pulses per predetermined drive distance are added to the accumulated direction and displacement to calculate the lens drive target in order to drive the photographing lens. Accordingly, a similar effect to that of the sixth aspect is attained.
An eighth aspect of the present invention comprises drive means 205A which does not drive the photographing lens in accordance with the current lens drive target when the direction of drive of the current lens drive target calculated by the second drive distance calculation means 207A is different from the direction of drive of the previous lens drive target.
In accordance with the eighth aspect, the photographing lens is not driven in accordance with the currently calculated lens drive target when the direction of drive of the currently calculated lens drive target is different from the direction of drive of the previous lens drive target. Accordingly, the photographing lens is driven smoothly.